1. Field of the Invention
The present invention relates to minimizing defects in components produced by lithography, particularly to the mitigation of substrate defects in reticles or masks utilized in extreme ultraviolet lithography, and more particularly to the use of a multilayer buffer layer deposited intermediate a reticle substrate and a reflective coating for mitigating substrate defects in reticles.
2. Description of Related Art
Extreme ultraviolet lithography (EUVL) systems are being developed for the production of electronic components formed on wafers via reflected radiation. The EUVL systems include reticles or masks that must be essentially free of defects that will print at the wafer, thus producing defective components. The reticles, for example, may be fabricated by depositing highly reflective multilayer coatings, such as Mo/Si, on superpolished substrates. Any localized structural imperfections on the reticle substrate may nucleate and evolve during the multilayer coating process into a defect that perturbs the reflected radiation field sufficiently to print at the wafer. Thus, there has been a need for mitigating the effect of small particle contaminants on the surface of the substrate that would nucleate a defect in the reflective coating.
The reticle defect problem may be divided into two components. First, there are the defects associated with the condition of the reticle substrate. These are particles, pits, or scratches on the reticle substrate that nucleate a growth defect in the multilayer coating. Second, there are the defects that are introduced during or after the multilayer coating process, which are particle contaminants that are embedded within or are sitting on the top surface of the coating. A low defect multilayer coating technology based on ion beam sputtering has been developed so that the coatings now being deposited are essentially defect-free, thus the greater risk is the starting conditions of the reticle substrate.
Modeling has been carried out that shows imaging of growth defects nucleated by spherical particles. The results have indicated that particles as small as 25 nm in diameter will nucleate defects in multilayer coatings, which can image at the wafer. Hence, all particles of a size greater than 25 nm must be removed from the reticle substrates prior to the deposition of the reflective coating. Currently, removal of the particles from the reticle substrates is carried out by cleaning processes that are expected to be ineffective for the removal of particles of less than about 60 nm, particularly since verification that such small particles exist is difficult.
Prior efforts to resolve the reticle substrate defect problem involved a single layer buffer-layer, see K. B. Nguyen, et al., J. Vac. Sci. Technol. 11:2964 (1993), where a 200 nm thick single-layer amorphous silicon (a-Si) buffer-layer was deposited on the substrate prior to a Mo/Si multilayer deposition in an attempt to smooth out lithographically defined steps (defects) on a silicon surface. This resulted in some reduction in the defect height and the transition at the edges of the step was made less severe by the buffer-layer, which is advantageous. However, the surface roughness of the Si was increased significantly (from 0.2 nm to 0.7 nm), making this process impractical for EUV lithography.
Currently, there is a process having the capability of depositing a-Si approaching 200 nm in thickness with much lower roughness than that observed by Nguyen, referenced above. However, the large stress typically found in smooth, single-layer films like a-Si can limit the applicability of this single-layer approach for the buffer layer.
As an alternative to cleaning and single-layer buffer layers, the present invention mitigates the effects of these small particles by depositing a multilayer film as a buffer layer in between the substrate and the reflective coating. The purpose of this buffer layer is to reduce the perturbation of the reflective coating due to the particles, pits, or scratches on the substrate.